Professional networking websites

What are your opinions on professional networking websites? Are you a member of any?

I'm a member of several professional networking websites, both for scientists and general professionals. What I find best of all of those websites is what I call "marking your presence in the internet". Before I joined those sites searching for my name revealed only some obscure websites, now first results are my professional profiles. Second good thing is that I can search for people with similar research interests or laboratory skills and this way find places where I could apply for a job etc. (more rarely - I can be found by recruiters and get offer).